Nowadays, massive growth in mobile broadband data traffic drives the evolution of the cellular systems to meet the tremendous need for increased capacity. However, the traffic growth will be difficult to address with spectrum currently available for cellular systems, especially for the valuable bands with low propagation loss below 6 GHz. The licensed band is superior, from the quality and availability perspective, but scarce and expensive, especially below 6 GHz. Given the fact that the current amount of unlicensed spectrum is comparable to the amount of licensed spectrum, and the fact that in some regions, more unlicensed spectrum is planned to be allocated, it is natural for operators to look into the potential of utilizing unlicensed spectrum to meet the traffic growth demands.
In 3GPP, studies on supporting LTE in unlicensed spectrum are likely to take place, focusing on low-power nodes. The basic set-up is “licensed-assisted access” (LAA) with carrier aggregation (CA) between licensed and unlicensed carriers. A licensed LTE carrier is used for all mobility, control signaling and parts of the user data, while one or more carriers in unlicensed spectrum are used to boost the user-data performance. For release 13, DL data transmission in unlicensed band is supported and UL is also discussed. When coming to release 14, UL data transmission in unlicensed band will be supported as well. Besides, unlicensed operation is important for NX systems as well, especially standalone deployment with both DL and UL to support enterprise solutions.
The idea of outer loop link adaptation (OLLA) is that estimated CQI is adjusted from reported CQI according to HARQ feedback due to that the estimated CQI may be outdated or inaccurate. In particular, outer-loop CQI adjustment based on block error rate (BLER) is employed to compensate the difference between SINR indicated by CQI and actual SINR. The basic idea is that whenever a NACK is received, the estimated SINR will be reduced by a pre-defined value (delta1), while after a few of continuous ACKs (M) being received, the estimated SINR will be increased by another predefined value (delta2). The value M and delta1 and delta2 depend on the targeted BLER. In fact, there are many versions of OLLA algorithms for a better or different performance.
In cellular systems operation in unlicensed band, there are two steps for accomplishing UL data transmission. First, UL grant transmission from BS allows a UE to know at which resource it should transmit data. Second, the UE transmits UL data in the granted resource. Note that the same UL resource could be allocated to one or more UEs a possible better efficiency when with listen-before-talk (LBT) mechanism.
For the second step for UL data transmission, listen-before-talk is needed at UE side to avoid collision. In other words, UL transmission is subject to listen-before-talk success. FIG. 1 shows an example where the channel clearance assessment (CCA) period (Green part) is used for UE to perform LBT. When channel is being sensed busy in the first UL granted sub-frame, UL data transmission is canceled. Then if a channel is idle in the second sub-frame, UL data is transmitted from UE side. In other words, the UL data transmission may not happen even when UL grant is received at UE side.
FIG. 2 shows an example of evaluation result for LAA-LTE and Wi-Fi coexistence. As shown in FIG. 2, the ratio of cancelled LAA UL is possibly large especially when the offered traffic is high. It means there is a large probability that no UL data transmission occurs even after an UL grant is sent from BS side.
FIG. 3 shows an example scenario in which one DL grant may be issued for a possible DL Transmission at a time duration shortly after the UL grant resource opportunity. In the operation as shown in FIG. 3, an eNB may firstly detect and decode in the first several symbols of granted subframe(s). If CRC identifies an ongoing UL data, the eNB proceeds to receive the rest of the UL data transmission; otherwise, the eNB starts DL LBT procedure to initiate a DL transmission specified by the DL grant.
However, data transmission in the UL grant resource has a high probability that it cannot be transmitted due to the LBT requirement which results in problems for a regular link adaptation. In particular.                For UL data transmission in UL grant resource as described above. UE performs listen before talk (LBT) at the beginning of UL grant resource and the data cannot be transmitted if channel clearance and random back-off procedure has not been completed before the granted time duration starts.        For DL data transmission in overlapping UL grant and DL grant as described above, the data cannot be transmitted: (1) if the eNB identifies an UL data TRANSMISSION is ongoing; or (2) if the eNB performs LBT and cannot complete the channel clearance detection and backoff procedure before granted opportunity starts.        
When the data is not transmitted in the grant resource, the target receiver can't decode data and misunderstand that the channel becomes worse so that the data is lost. According to OLLA algorithm, the eNB will decrease MCS level for DL or UL transmission. Since the MCS decreasing is always more responsive to a TRANSMISSION failure than its increasing step to a TRANSMISSION success and the ‘no transmission’ situation become more often in aforementioned unlicensed spectrum cases, the selected MCS would be always in a low level even when the channel status is good. Thus, the spectrum efficiency is low.